The fishery for Rastrineobola argentea in Lake Victoria: estimation of potential yields using a new approximate model based on primary production

Abstract

Sustainable fishery yields of Lake Victoria dagaa, Rastrineobola argentea, a silvery zooplanktivorous cyprinid, are estimated in the absence of catch/effort and survey data. The starting point is an estimate of sustainable yield of a fish in another large lake in the region, the baseline lake. This is used to estimate the potential yield of a fish occupying an analogous niche in a second lake, the target lake. Several baseline lakes may be used to provide results for the target lake. A new approximate model scales fish yield in baseline and target lakes according to the ratio of the logarithm of primary productivity. We modify this model to account for differences in productive capacity in the two taxa by scaling the result according the ratio of Φ′ values, the auximetric growth parameters. Here we employ sustainable fishery yields of freshwater sardines, Limnothrissa and Stolothrissa, from Lakes Tanganyika and Kariba as the baselines for the model. For Lake Victoria dagaa, the validity of linking primary production and zooplanktivore production one trophic level removed in the food web is considered a reasonable approximation. The principal uncertainties in applying the model are evaluated and estimated quantitatively using a bootstrap procedure derived from error distributions of the model input parameters. Rastrineobola population biomass estimated from the model's fishery yield results is very similar to previously published values. The potential sustainable yield of 5.18–8.14 t km −2 in relation to recorded 1990 landings of 1.24 t km −2 (84 000 t over the whole lake, and the latest data available to us) suggests that catches may be safely doubled with minimal risk. Although this finding is sensitive to the accuracy of the catch records, it is in accord with the small number of other estimates.